In recent years the requirements for a liquid crystal projector have been that it provide a sufficiently bright image so that a room in which it is used does not have to be darkened in order to view the projected information, that the liquid crystal projector be portable as a result of being compact and lightweight, and that the liquid crystal projector have a high resolution. Such a projector places special demands on the lens that is used to project the light from the liquid crystal display, as the lens must provide a bright image, have excellent resolving power, and yet be compact. Because a projection lens for a color liquid-crystal projector needs to have a beam-combiner on its reducing side for synthesizing the three component colors which are combined to form the projected information, the projection lens must have a large back focus. Further, the zoom lens should be substantially telecentric.
As a zoom lens for such a liquid crystal projector, Japanese Laid Open Patent Application H10-20192 is known. However, in this zoom lens the brightness of the image is not sufficient as described above, since the F.sub.NO (being near 2.3) is not sufficiently low to enable projected information to be viewed without the room being darkened. Furthermore, the aberrations of this zoom lens are not sufficiently well-corrected so as to provide the image resolution required. The requirement that the zoom lens simultaneously provide a high resolution and have a small F.sub.NO make the design of such a lens more difficult, since the focal depth becomes smaller as the F.sub.NO becomes smaller.
As a zoom lens having sufficiently well-corrected aberrations to be used in such a projector, Japanese Laid Open Patent Application H10-186235 is an example. This zoom lens consists of five lens groups, of which three lens groups move during zooming. However, even in this zoom lens, the brightness of the projected image is not sufficient so that it can be used to project images in a room that has not been darkened. Also, having three lens group that move make the lens more complex, and thus make it difficult keep the lens lightweight and of low cost.